“Constrained Geometry” Dialkyl Catalysts. Efficient Syntheses, C−H Bond Activation Chemistry, Monomer−Dimer Equilibration, and α-Olefin Polymerization Catalysis

Abstract

This contribution reports an efficient synthesis of the “constrained geometry” group 4 dibenzyl complexes Me2Si(η5-Me4C5)(tBuN)MR2 (CGCMR2, where R = CH2Ph; M = Ti (1), Zr (2)), as well as the substantially different reaction patterns in the cocatalytic activation of the R = CH2Ph and Me complexes with B(C6F5)3, PBB (tris(2,2‘,2‘‘-perfluorobiphenyl)borane), and Ph3C+B(C6F5)4-. The resulting cationic complexes are highly but not equivalently active for α-olefin polymerization and copolymerization catalysis. The reaction of the neutral free ligand CGCH2 with Ti(CH2Ph)4 in aromatic or saturated hydrocarbon solvents at 60 °C cleanly affords 1 in 90% yield, while the corresponding reaction with Zr(CH2Ph)4 produces 2 in lower yield. When activated with Ph3C+B(C6F5)4- at low temperatures, 2 generates cationic CGCZrCH2Ph+ B(C6F5)4- (4). However, unlike the corresponding metallocene dibenzyl, the cationic derivative of which (Cp2ZrCH2Ph+B(C6F5)4- (3)) can be isolated in quantitative yield, the reaction of 1 with B(C6F5)3 and Ph3C+B(C6F5)4- affords intramolecular C−H metalation products Me2Si(η5,η1-C5Me3CH2)(tBuN)Ti+[ηn-PhCH2B(C6F5)3]- (5) and Me2Si(η5,η1-C5Me3CH2)(tBuN)Ti+B(C6F5)4- (6), respectively. In contrast, the reaction of CGCTiMe2 with B(C6F5)3 cleanly generates CGCTiCH3+CH3B(C6F5)3- (8) without C−H bond activation as well as dinuclear [CGCTiMe(μ-Me)MeTiCGC]+MeB(C6F5)3- (11), which is in equilibrium with 8 and CGCTiMe2 (ΔG298K = 1.3(2) kcal/mol in favor of 8). The reaction of CGCTiMe2 with sterically encumbered PBB and Ph3C+B(C6F5)4- yields predominantly cationic dinuclear species, and analytically pure [CGCTiMe(μ-Me)MeTiCGC]+[MePBB]- (9) can be isolated in quantitative yield. Complexes 5 and 6 are highly active homogeneous catalysts for ethylene and propylene polymerization, producing ultra-high molecular weight (Mw > 106) polyethylenes with high melting transition temperatures (Tm = 142 °C), as well as syndiotactic-enriched atactic polypropylenes having appreciable molecular weights. Although C−H bond-activated complexes 5 and 6 are ineffective for ethylene and 1-hexene copolymerization, the CGCTi(CH2Ph)2/MAO system is highly active at 60 °C to incorporate 1-hexene in large quantities (69.9%). Finally, comparisons of polymerization catalysts bearing different counteranions at various temperatures demonstrate the substantial influence of anion identity on α-olefin polymerization activity, catalyst stability, and product polymer microstructure.